I got this error whilst trying to get my head around the templating system. I’ve no idea what caused it but I’ve got a quickfix/hack for those who need to get mage back up:
Just put a simple
around the calls to getParentBlock (line 42 and 49 i think) in app/code/core/Mage/Checkout/Block/Links.php
after that, you may get a similar error from app/code/core/Mage/Wishlist/Block/Links.php. Just do the same around the getParentBlock call
I hope this helps.